Antiaromaticity to Aromaticity: From Boroles to 1,2‐Azaborinines by Ring Expansion with Azides

Abstract

We have exploited the reactivity of antiaromatic boroles, gaining access to aryl-substituted monocyclic 1,2-azaborinines. The observed ring-expansion reaction of inherently electron-deficient boroles with organometallic and organic azides is demonstrated for representative examples. This substance class is expected to provide a new avenue into 1,2-azaborinine chemistry, especially in the area of functional organoboron materials. Our results are based on NMR and UV/Vis spectroscopy as well as single-crystal X-ray crystallography and provide a virtually quantitative approach that also offers numerous points of variation.

Scope of the Thermal Ring‐Expansion Reaction of Boroles with Organoazides

“…It is interesting to note that the π‐electron‐donating and ‐accepting abilities of the substituents on boron are no longer strongly reflected in the 11 B NMR chemical shifts of the six‐membered rings, as is the case for the boroles. However, the effect is still discernible in the series of 1,2‐azaborinines, which differ only in their boron substituents (R 1 =SiMe 3 ): δ ( 11 B)=41.4 ppm (R 2 =Mes), δ ( 11 B)=39.8 ppm (R 2 =Ph), and δ ( 11 B)=37.4 ppm (R 2 =C 4 H 3 S, 9 ). The influence of the substituents on the 11 B NMR resonances is consistent with the trend observed for the corresponding borole structures .…”

“…As indicated by the sums of the bond angles about B1 and N1 of 360.0° and 359.9°, respectively, the heteroatoms adopt almost perfectly trigonal‐planar coordination geometries. The molecular structure of 9 could also be obtained, but a disorder in the heterocycle, which is frequently encountered for these types of structures, prevented a more accurate analysis of the metric parameters. Nevertheless, it is interesting to note that all aryl substituents, including the thienyl group on boron, are arranged in a propeller‐like fashion around the central ring, indicating no significant conjugation between them .…”

“…The reaction of boroles, five‐membered unsaturated boron heterocycles, with organic azides is known to provide access to 1,2‐azaborinine derivatives . By formal nitrene insertion, antiaromatic boroles are thereby transformed into aromatic azaborinines containing a boron‐nitrogen bond.…”

O,N,B-Containing eight-membered heterocycles by ring expansion of boroles with nitrones

“…Depending on the steric demand of both reaction partners, either insertion of the α-nitrogen atom during a de-dinitrogenative pathway to afford six-membered 1,2-azaborinines is observed (Fig. 1, I), 36 or incorporation of the terminal N-atom of the azide into the borole ring occurs, resulting in 1,2-azaborinine-substituted azo dyes ( Fig. 1, II).…”

Electronic Push–Pull Modulation by Peripheral Substituents in Pentaaryl Boroles

“…Being an isolable antiaromatic compound class, boroles have received growing interest in recent years . Due to their peculiar electronic situation, they feature intriguing reactivity including E−H bond splitting, cycloadditions and ring expansions, as well as Lewis acidity enabling adduct formation even with weak Lewis bases such as CO …”